The internal impedance of a battery may vary during use. As a battery ages and experiences charging and discharging cycles, the battery may physically and/or chemically change in ways that increase the internal impedance of the battery, thereby degrading the battery, or diminishing its state of health. Eventually, the internal impedance may increase to a point where the battery has reached the end of its useful life.
Additionally, for any given state of health of the battery, the internal impedance may vary depending on various aspects of the circumstances in which it operates. For example, the temperature of the battery, the state of charge of the battery (i.e., how much electrical energy the battery has stored), the magnitude of electric current being discharged from or supplied to the battery, and whether the battery is charging or discharging may all affect the internal impedance.
Having an accurate real-time estimate of the internal impedance of a battery may prove useful for a number of reasons. The internal impedance of a battery affects the maximum amount of current the battery can discharge and the maximum amount of charging current the battery can receive without driving the voltage at the battery's terminals below a minimum acceptable level or above a maximum acceptable level. The greater the internal impedance, the less discharging or charging current the battery can tolerate. Accordingly, the amount of power that a system can draw from the battery and the rate at which the system can recharge the battery may vary depending on the present internal impedance of the battery. Thus, having an accurate estimate of the present internal impedance of the battery may facilitate effectively and efficiently controlling the discharging and charging of the battery, as well as controlling other aspects of the operation of the power system to meet varying power needs. Furthermore, the internal impedance of the battery may serve as a reflection of a state of health, or degree of degradation, of the battery.
The “FreedomCAR” manual published in October 2003 by the Idaho National Engineering & Environmental Laboratory discloses procedures for estimating the internal resistance of a battery. For example, the FreedomCAR manual discusses a hybrid pulse power characterization test (“the HPPC test”). The HPPC test disclosed by the FreedomCAR manual involves identifying the voltage and current of a battery at various points in a discharging and charging cycle and estimating the internal resistance based on the identified values. The charging and discharging cycle involves discharging the battery at a constant current for a 10 second discharging period, refraining from charging or discharging the battery for a 60 second rest period, and then charging the battery at a constant current for a 10 second regeneration period. Battery voltage is measured at the beginning of the discharging period, at the end of the discharging period, at the end of the rest period, and at the end of the regeneration period. Subsequently, these voltages and the known magnitudes of the charging and discharging currents are used to calculate estimated values of the internal resistance of the battery during the discharging period and the charging period.
Other publications also discuss methods for estimating the internal impedance of a battery. For example, Published U.S. Patent Application No. 2007/0145953 to Asai et al. (“the '953 application”) also discusses methods for estimating the internal impedance of a battery. The methods disclosed by the '953 application take battery temperature and battery degradation into consideration when estimating the battery's internal impedance. Similarly, Published U.S. Patent Application 2007/0013347 to Kamohara (“the '347 application”) discusses methods of estimating internal impedance of a battery. The '347 application discloses that its methods include correcting an estimated internal impedance of a battery based on a sensed temperature of the battery and a map of a relationship between internal impedance and battery temperature.
Published U.S. Patent Application No. 2011/0077879 to Paryani (“the '879 application”) employs many of the teachings of the FreedomCAR manual, the '953 application, and the '347 application to estimate internal impedance of a battery. The '879 application discloses periodically testing the impedance of the battery when the battery is not needed to provide power to loads. Specifically, the '879 application suggests that the impedance testing is performed when the battery is being charged at night. The process includes charging the battery to a predefined state of charge (e.g., 60% SOC), refraining from charging or discharging the battery for a relaxation period, and then resuming full-current charging for a period. Battery voltages are identified at the end of the relaxation period and at the end of the final charging period. Similar to the procedure disclosed in the FreedomCAR Manual, the procedure disclosed in the '879 application uses the known magnitude of electric current during the final charging period in combination with the measured voltages at the end of the relaxation period and the final charging period to calculate an impedance of the battery. The '879 application suggests using this measured impedance value of the battery to determine an impedance degradation factor representative of a change in the battery's impedance due to degradation of its state of health.
Just as the prior '953 application discloses taking battery temperature and battery degradation into account when estimating battery impedance, the '879 application indicates that, after its system determines the battery impedance in a controlled charging situation at night, it may subsequently estimate internal impedance during use in a manner that accounts for battery temperature and battery degradation. To do so, the '879 application discloses multiplying the impedance degradation factor determined during the testing procedure by a reference impedance. Similar to the disclosure in the '347 application of using a map to correct impedance based on battery temperature, the '879 application suggests using a look-up table responsive to state of charge and battery temperature to establish the reference impedance.
Although the '879 application discloses a method for measuring internal impedance when the power system is inactive at night and subsequently estimating internal impedance based on battery degradation, battery temperature, and state of charge, certain disadvantages may persist. For example, the '879 application's reliance on impedance measurement when the power system is inactive may have drawbacks. Limiting impedance measurement to situations when the power system is inactive may result in undesirably infrequent impedance measurements and/or impinge on an operator's ability to freely use the power system as desired.
The system and methods of the present disclosure may solve one or more of the problems set forth above.